Morphological identification and functional analysis of central neurons innervating the pens retractor muscle of Helix pomatia

Abstract

1. 1. Cobalt retrograde axonal diffusion and intracellular recordings were used to identify central neurons innervating the penis retractor muscle (PRM) of Helix pomatia. In intact brain-muscle preparations we studied the spontaneous activity of identifiable central neurones and the peripheral effector suspected to be innervated by them simultaneously. 2. 2. Central neurons sending off processes into the penis retractor nerve (PRN) were located in the cerebral ganglia, the right cerebro-pedal connective and the pedal ganglia. Four neurons located on the latero-ventral surface of the right pedal ganglion have been morphologically and electrophysiologically identified revealing the possibility of a routine recognition of individual central neurons from animal to animal. 3. 3. The four identifiable neurons could be classified by virtue of their electrophysiological properties in three types: (1) two irregular bursting cells, (2) one regular beating cell and (3) one silent cell. The firing patterns of the three active cells were found to correspond with rising phase resp. tension maintenance or relaxation of tonic contraction within the PRM. 4. 4. The two irregular bursting cells (IB cells) could be distinguished in IB cell 1 and IB cell 2 by their different burst activities in relation to the muscle behaviour. Spontaneously occurring or' intracellularly stimulated burst activities of IB cell 1 showed a correlation in time with the rising phase of strong contractions within the PRM. The bursting activities of IB cell 1 were not impaired after disconnecting brain and muscle, while the tonic contraction within the PRM was abolished. 5. 5. Spike discharge of the second irregular bursting cell (IB cell 2) accompanied muscle relaxation. Intracellular stimulation of IB cell 2 caused burst activity but failed to produce relaxation from muscle tone. After disconnecting brain and muscle IB cell 2 became silent. 6. 6. Increase in the regular beating rhythm of the third active cell could be observed during tonic muscle activity mainly during the maintained state of muscle tone. After disconnecting brain and muscle this cell fired the whole time in a very regular manner. 7. 7. From these studies it is suggested that IB cell 1 involved in generating strong contractions within the PRM. IB cell 1 seems to exert its effect directly on peripheral nervous structures without the use of centrally located interneurons possibly by a modulatory effect on the efficiency of peripheral motoneurons. I B cell 2 and the regular beating cell appeared to receive direct or indirect afferent input from the peripheral organ. The role of the silent is as yet unresolved.